It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A-B and A-B-A
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a selectively hydrogenated conjugated diene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics. When the content of the alkenyl arene compound is small, the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the blocks A are thermodynamically incompatible with the blocks B resulting in a rubber consisting of two phases; a continuous elastomeric phase (blocks B) and a basically discontinuous hard, glass-like plastic phase (blocks A) called domains.
Since the A-B-A block copolymers have two A blocks separated by a B block, domain formation results in effectively locking the B blocks and their inherent entanglements in place between the A blocks and forming a network structure. These domains act as physical crosslinks anchoring the ends of many block copolymer chains while the copolymer is at a temperature below the glass transition temperature of the domains. The copolymer is easily processed while above the glass transition temperature of the domains which permits recycle of scrap unlike vulcanized or chemically crosslinked polymers. Such a phenomena allows the A-B-A rubber to behave like a conventionally vulcanized rubber in the unvulcanized state after melt processing and is applicable for various uses. For example, these network forming polymers are applicable for uses such as moldings of shoe sole, etc.; impact modifier for polystyrene resins and engineering thermoplastics; in adhesive and binder formulations; modification of asphalt; etc.
Conversely as the A-B block copolymers have only one A block, domain formation of the A blocks does not lock in the B blocks and their inherent entanglements. Hence, these diblock copolymers are referred to as non-network forming polymers. Moreover, when the alkenyl arene content is small resulting in a continuous elastomeric B phase, the strength of such polymers is derived primarily from the inherent entanglements of the various B blocks therein and to a much lesser extent the domain formation by the A blocks therein. However, the non-network forming polymers have found particular utility as viscosity index improvers (U.S. Pat. Nos. 3,700,748; 3,763,044; 3,772,196; 3,965,019; and 4,036,910). Non-network forming block copolymers are also utilized in adhesive and binder formulations and as modifiers or plasticizers for polystyrene resins and engineering thermoplastics (U.S. Pat. No. 4,584,338).
Network forming copolymers with a high alkenyl arene compound content, such as more than 70% by weight, provide a resin possessing both excellent impact resistance and transparency, and such a resin is widely used in the field of packaging. Many proposals have been made on processes for the preparation of these types of block copolymers (U.S. Pat. No. 3,639,517).
Both the network forming (A-B-A) and non-network forming (A-B) polymers may be handled in thermoplastic forming equipment and are soluble in a variety of relatively low cost solvents.
While in general these block copolymers have a number of outstanding technical advantages, one of their principal limitations lies in their sensitivity to oxidation. This behavior is due to the unsaturation present in the elastomeric section comprising the polymeric diene block. Oxidation may be minimized by selectively hydrogenating the copolymer in the diene block, for example, as disclosed in U.S. Pat. No. Re. 27,145 and the above referenced VI improver patents. For example, prior to hydrogenation, the block copolymers have an A-B or an A-B-A molecular structure wherein each of the A's is an alkenyl-arene polymer block and B is a conjugated diene polymer block, such as an isoprene polymer block or a butadiene polymer block preferably containing 35-55 mole percent of the condensed butadiene units in a 1,2 configuration.
Non-network forming (A-B) block copolymers are especially deficient in applications in which good mechanical integrity and deformation resistance are required. This behavior is a consequence of the lack of inherent entanglements of the various B rubber blocks and to a much lesser extent the domain formation of the A blocks therein which controls strength under tensile deformation.
Network forming copolymers are known to have particularly high tensile strengths at room temperature due to the formation of glassy phase arene block domains which act as physical crosslinks locking in the inherent entanglements within the rubbery B block matrix. The mechanical integrity of these domains and the resulting network structure appear to control the tensile strengths of these copolymers. Moreover, at elevated temperatures, the mechanical integrity of block copolymers is limited to the integrity of the hard phase arene block domain. For example, network forming copolymers having arene blocks of polystyrene have poor mechanical properties at high temperature which may be attributed to the weakening of the polystyrene domains above the polystyrene glass transition temperature (Tg) of 100.degree. C. Improvements in the high temperature characteristics of the network forming block copolymers may be achieved by enhancing the integrity of the alkenyl arene domains to higher temperatures.
U.S. Pat. No. 4,868,245 teaches that substantial improvement in the high temperature capabilities of the block copolymer can be obtained by increasing the arene A block glass transition temperature (Tg), and by extending the mechanical integrity of the arene A block domains to higher temperatures. These performance characteristics are accomplished by grafting functional groups to the arene blocks, A, without substantially modifying the elastomeric B blocks. The high temperature properties are improved by grafting carboxyl functional groups in an all acid form, in a combination of their acid and neutralized metal carboxylate salt forms, or in an all neutralized metal carboxylate salt. Furthermore, the high temperature properties are also improved by increasing the degree of carboxyl functionality and/or by utilizing metal ions of increasing ionized valence states.
The glassy phase arene block domains in both the network and non-network forming polymers have a dramatic effect on the melt viscosity at temperatures in excess of the glass transition temperature of the arene A block. In order to render these materials more processable, other components are added such as processing aids which lower the viscosity and soften the polymer as described by Crossland et al. in U.S. Pat. No. 3,827,999; Gergen et al. in 3,865,776; and Hendricks et al. in Britain Patent No. 1,160,198. The mechanical properties of the block copolymers are adversely altered in these polymer blends.
The combination of non-functionalized, elastomeric block copolymers and extender resins that are compatible with the conjugated diene blocks is known such as described in U.S. Pat. Nos. 3,830,767; 3,827,999; and 3,485,787.